Project

AIM OF THE PROJECT: Implementation and analysis of BAJA All-Terrain Vehicle(ATV) with Continuous Variable Transmission(CVT) at Four different scenarios in Matlab Simulink.

OBJECTIVE: To run a system level simulation of BAJA-ATV as a prerequisite so as to observe the characteristics of the system at four different situations which are as follows:

            a) Open-Loop system with CVT given from a Signal Builder.

            b) Open-Loop system with Manual Control and CVT given from a Signal Builder.

            c) Closed-Loop system with CVT given from a Look-up Table relationship.

            d) Closed-Loop system with Manual Control and CVT given from a Look-up Table relationship.

INTRODUCTION:

ATV known as All-Terrain Vehicle are a typical kind of motorized vehicle specially designed to travel at any rough and shallow position with the help of four non-pneumatic or low pressure tires. In this project the type of ATV used is designed specifically for the event organised by BAJA. The seats are so designed to be straddled by the operator along with handlebars for steering control. According to the American National Standards Institute(ANSI), this ATVs are also termed as quad bike with a single operator but nowadays due to modification operators are allowed to take a single passenger and those type of ATVs are classified as Tandem ATVs. The engine used in the vehicle is provided by BAJA with a specifications of around 340cc and 11HP. The objective of BAJA is to design and implement off-highway vehicles that can smoothly transmit in extremely rough terrain for emergency situation where normal on-road vehicle cant able to reach.

Another type of ATV organised by BAJA is used as an Collegiate Design series which comes under the name BAJA SAE thats run by the organisation named as Society of Automotive Engineers International(SAE International) where students of various universities accross the world can able to participate were they have to build their own prototype with an engine specification not going beyond 305cc and 10bHP. 

Major Components of BAJA-ATV Vehicle:

1) Continuous Variable Transmission(CVT): Nowadays this is the most popular transmission method that are getting implemented in the BAJA vehicle. Unlike the Automatic Transmission(which uses a hydraulic system to sense the pressure that is being created in proportional to the speed and gradually changes the gear without any input from the driver) CVT uses a pair of pulleys of which one is connected to the engine and the other one is connected to the wheel while the two pulleys are connected with the help of a belt. 

So when the circumference of the belt(Effective Diameter) in any of the Pulley increases then gradually the speed will get decrease, if this is the situation for the Driver pulley then the system is running at High gear mode whereas if its for the Driven pulley then the system is Low Gear mode.

   

Here the Left Pulley is the Driver/Primary Pulley and the Right Pulley is the Driven/Secondary Pulley.

From the above figure we can see that the pulleys consists of two cone-shaped halves that moves closer or away from each other. The belt in the form of V-Shape runs between the two halves, the effective diameter is always proportional to the distance between the two halves of the pulley. The V-shaped nature of the belt allows it ride over one half of the pulley and fall below other half of the pulley therefore changing the Gear-ratio of the pulley according to it. Now as the distance between the belt and the pulley is kept intact, so the pulleys must be adjusted in such a way(one getting bigger and the other getting smaller) that the tension on the belt is always gets preserved. The typical efficiency of CVT is around 88% which allows the system to run at most efficient RPM regardless of the vehicle speed, it also allows the system to achieve greatest possible power by changing the effective diameter of the Driver/Primary pulley.

The crucial advantage that makes CVT more popular than other transmission is because of these Belt-Pulley arrangement it can provide numerous number of Gear ratios which means whatever may be the engine speed it can change the Gear ratios according to it and thereby operate the system at maximum efficiency. This unique ability can circumvent the consumption of fuel to a great extent and therefore making the vehicle economically suitable and effective. 

In this project, there is a CVT subsystem implemented in MATLAB-Simulink which is used to perform the above function along with the generation of the nature of curve as an output from the both the Primary and Secondary Pulley. The subsystem is shown below:

Here the block that is performing the CVT function is the Variable-Gear Ratio where 'r' denotes the gear-ratio at every instance of time between the input which is the Base(B)/Primary Pulley or Driver Pulley and the output which is the Follower(F)/Secondary Pulley or Driven Pulley. In this project, the input to the 'r' is given either in the form of Signal Builder or through Look-up table format. Now other than that, the two Rotational Motion Sensor block is implemented to constantly track and monitor the nature of curve(in RPM) between the CVT Primary and CVT Secondary Pulley. The Inertia block is connected to provide the Rotational Motion into the system and that will be controlled by the Variable-Gear ratio.  

2) Vehicle Body: This is the most important aspect of a vehicle whose two main functions are: one- to provide rigid support and protection to all other delicate parts of the vehicle like engine section, trunk etc. two- to counter static as well as dynamic loads thereby saving other parts from any sorts of deflection and distortion.

  

In this project, the vehicle body is represented in a different subsystem which is required to define not only the vehicle body but also the tires specifications along with the Shoe-Brake. The subsystem implemented in the Matlab-Simulink is shown below:

The above subsystem contains various blocks so now let just get a brief idea about each of these blocks used above:

• Vehicle Body Block: It represents a two axle longitudinal vehicle body as shown below:

    

Here, 'H' is the mechanical translational port that determines the horizontal motion of the vehicle body.

         'V' is the output port through which the vehicle velocity can be checked.

         'NR' and 'NF' are the output port that stands for the forces acting on the Rear and Front normal wheels.

         'beta' constitute the inclination of the road through which the vehicle will move.

         'W' stands for the velocity of the wind acting opposite to the body.

Other than that there are few parameters which are needed to be specified like aerodynamic drag, center of gravity, external mass etc. which we have taken as given below:

• Tires Block: This block represents longitudinal motion tires that generally performed its function using a Magic formula which is nothing but an emperical equation of Four coefficients that can be defined by the user. 

 

Here, 'N' is the input port through which the Force is applied to the tires.

         'A' stands for the rotational motion applied to the wheel axle.

         'H' represents the translational conserving port through which the thrust is applied to the vehicle body.

         'S' is the slip representing in the tire.

Thus the properties of the tires depends on the Four co-efficients whose values are defined as shown below:

 

• Double-Shoe Brake: This block represents the braking action of the vehicle. It consists of two pivotal rigid shoes that generally press against a rotating drum to de-accelerate the wheel.

Here, 'F' represents the force applied in the form of brake to de-accelerate the drum.

         'S' stands for rotational motion given to the drum-shaft.

Along with this it is necessary to define other parameters like drum radius, shoe span angle, etc, of the brake. The values of the parameters are taken as below:

3) Generic Engine: This is a basic internal combustion engine block that belongs to spark-ignition and diesel type. The speed and the torque is mentioned according to our need and according to the threshold value of the throttle the crankshaft speed is taken into consideration. The below figure shows the Generic Engine block of Matlab Simulink:

Here, 'T' is the throttle input signal that ranges between 0 to 1 that specifies the amount of torque given to the engine.

         'B' and 'F' represents the rotational conserving ports relating to engine base and crankshaft.

         'P' and 'FC' are the output ports through which the Power and the Fuel Consumption of the engine can be analyse.

The parameters that are required to define for setting up the Generic engine block is as follows:

In this project, the Speed vector and the Torque vector is taken as a tabulated format from Workspace according to the relationship between them as shown below:

So according to this relationship, the Speed vector and the Torque vector datapoints are stored in a Matrix format in the Workspace which is as follows:

Ultimately this datas are used to specify the Speed and Torque of the Generic Engine.

4) Simple Gear: This block is to define the relationship of the Base and the Follower with a Fixed Gear ratio, we can also specify the condition whether the output shaft will rotate opposite/same direction w.r.to the input shaft. Along with that we can also add losses relating to Meshing and Bearing if required. 

It works on a basic mathematical relationship given as:

`GearRatio = (Number of Teeth in Driven Gear)/(Number of Teeth in Driver Gear)`

This is a Simple Gear block where 'B' and 'F' stands for the Base and the Follower conserving ports. The Gear Ratio and the condition are defined as follows:

5) Engine Sensor Subsystem: This subsystem is created to track and analyse the Generic Engine crankshaft RPM(F) in accordance to the throttle input. 

So the crankshaft speed is going as an input to the Engine Sensor which on the other hand is analysing the Engine RPM with the help of a Rotational Motion Sensor Block as shown below:

This is the Block present inside the Engine Sensor subsystem where using the Motion Sensor the engine speed is getting collected in the Scope. 

DEVELOPMENT AND ANALYSIS OF THE TARGET MODEL:

Scenario 1:

Study of BAJA ATV Open Loop Model with CVT given from a Signal Builder:

SIMULINK MODEL:

Here in this model the Brake and the Throttle Input in given as follows:

It can be seen that initially the throttle value was kept at 0.3 but after 20 sec it was increased to 1, whereas the Brake was kept at 0 for the entire running time.

Similarly, the CVT ratio is also given from a Signal Builder which is of this nature as shown below:

The value of the CVT ratio was kept at 2.3 for 4sec then after that it was reduced to 1.8 and ultimately get settled at this value from 35sec onwards upto 80 sec.

RESULTS AND DISCUSSIONS:

The Model was run for 200sec and the corresponding results are coming as an output:

This is the corresponding output of both the CVT Primary as well as the CVT Secondary pulley. As we know that the Primary Pulley is connected to the Engine so the nature of the Curve will be same for the CVT Primary RPM and the Engine RPM whereas the CVT secondary will come less in magnitude than the Primary because of the CVT Gear ratio which we are applying as an input.

In order to justify the Statement, from the Graph given above initially the Value of CVT Primary is 1600RPM and corresponding CVT Secondary is 693RPM after that both CVT Primary and CVT Secondary settling at a value of 3830RPM and 2130RPM respectively.

As we know, `Gear Ratio= (CVT Primary)/(CVT Secondary)`

So, `Gear Ratio = 1600/693=2.3`  - which is the same value as we given as an input to the CVT ratio for 4sec.

Again after setting, `GearRatio = 3830/2130=1.8`  - which is the same value we have given to the CVT ratio after 35 sec.

The corresponding Velocity Curve is coming as follows:

The most important thing to note here is that Velocity of the Vehicle Curve is coming exactly the same as the CVT Secondary(though there are changes in magnitude of the curve which is due to the Velocity of the vehicle is measured in km/hr whereas the CVT Secondary is in RPM) and it is because of the fact what we discussed earlier that the Secondary/Driven Pulley is always connected to the vehicle wheel. Now with respect to the throttle the Velocity increases when the value of the throttle increased from 0.3 to 1 and other than that we will not seen any changes as the Brake input is kept at zero for the entire span.

 

As we said earlier the curve corresponding to Engine RPM given above will come exactly same as that the CVT Primary RPM. From both the CVT Primary and the Engine curve we can see that both of them are getting settled at 3830 RPM and it can be justify from the Torque-Speed Curve point of view where the curve is getting stopped at a value around 3830RPM as seen from the below figure:

So this is the reason the curve of both the Engine RPM and the CVT Primary is settling at around 3830RPM.

Scenario 2:

Study of BAJA ATV Open Loop Model with Manual Control(Dashboard) and CVT given from a Signal Builder:

SIMULINK MODEL

Here both the Brake and the Throttle input is given manually while the Model is running from the Dashboard.

While the CVT ratio is kept same as shown above in the Scenario 1.

RESULTS AND DISCUSSIONS:

The above curve is correponding to the vehicle velocity. From here we can see that as we are decreasing the value of throttle input from 1 using the Dashboard knob the velocity is also decreasing from the inital value and in that situation after certain time the value of brake is increased from 0 to 0.3 which ultimately made the velocity curve to fall to 0, so after that even if we made the brake input to 0 or increase the throttle input it still wont have any change in the velocity curve.

The correponding CVT Primary and Secondary will come as following:

As discussed above in the Scenario 1, the CVT Secondary curve nature will be exactly same as that of the Vehicle velocity and both of them are falling to 0 when the brakes are applied.

Similarly, the curve nature of CVT Primary will be exactly the same as that of the Engine RPM whose output is shown below:

Here also the Engine Speed is falling to zero at exactly the same time when the brakes are applied just like other curves. This concludes the study of the Scenario 2.

 Scenario 3:

Study of BAJA ATV Closed Loop Model with CVT given from a Look-Up Table:

SIMULINK MODEL

Unlike the other two scenario, here the CVT Ratio is given in a Closed Loop format for constantly updating the data points of the CVT ratio as per the Look-Up table. Another thing to be noted is that the Throttle and the Brake is given as a Step input following exactly the same condition as discussed in the above two scenarios.

The condition given as a step input for the throttle is as follows:

From here we can see that initially the throttle was given a value of 0.3 then after 20 sec it was increased to 1 which is same as the other two scenarios.

Similarly, the condition given as a step input for the brake is as follows:

The brake input is always maintained at zero.

Now the CVT ratio is basically generated from the lookup table in the form of "Lookuptable_data.mat". It consists of CVT ratio as 'cvtr' and the vehicle speed as 'vehicle_speed'. It comes in the matrix format which is as follows:

Using these values the Look-up table which is basically the relation between the CVT gear ratio and the Vehicle Speed is coming as follows:

Here the CVT ratio is staying at 2.3 for around 20km/hr and after that it was settling at 1.8 from around 31km/hr onwards.

RESULTS AND DISCUSSIONS:

Using the above Look-up table the corresponding CVT ratio curve is coming as follows:

Here also we can see that the ratio is 2.3 for 4sec and after that it was falling to 1.8 at 35sec onwards upto 200sec. This is coming exactly the same for the above two scenarios.

Similarly, the CVT Primary and Secondary RPM is coming as shown below:

The corresponding velocity curve and the Engine RPM is coming as shown below:

This is coming exactly same as that of Open Loop Model(without using Dashboard) so there is nothing new to discuss here. Therefore, this is all about the Scenario 3.

Scenario 4:

Study of BAJA ATV Closed Loop Model with Manual Control(Dashboard):

SIMULINK MODEL

Just like Scenario 2, the Brake and the Throttle is controlled manually from the Dashboard using the knob. Here also the CVT ratio is given in a closed loop format using the Lookup Table as discussed in Scenario 3. 

RESULTS AND DISCUSSIONS:

The vehicle velocity curve along with the Throttle and the Brake is coming as follows:

Here we can see that initially when the Throttle input is increased from 0.3 to 1 then velocity also gets increases to a maximum value by keeping the Brake at 0. But after that we maintained the Throttle at 1 for the rest of the Simulation and produced three peaks in the Brake input with a maximum value of 0.5. During the peak we can see that the velocity curve is decreasing to a certain value and at the moment the peak is reducing to 0 the velocity curve is again moving back to its normal position. So as we given three peaks in the Brake input, the velocity curve shows the above nature exactly for the three times.

Similarly, the CVT Primary and CVT secondary RPM will follow the same nature as shown below:

Here also we can see the exact nature of the vehicle velocity curve followed by both the Primary and the Secondary pulley of the CVT. As we discussed earlier that the Engine RPM will follow the same nature of that of the CVT Primary. So the Engine RPM will also follow the same nature but that of the CVT Primary which is shown below:

So this concludes the Discussion of all the Four scenarios.

CONCLUSION:

From the Discussion of the Four Scenarios we can say that all the parameters like vehicle speed, engine RPM etc, of the BAJA ATV design model is following the exact same relationship at different CVT ratio implementation and thus we successfully analysed the model at various condition in resemblance with the practical scenario. We also analysed in detail the Output results in accordance with different nature of the Throttle and the Brake input.

REFERENCES:

1) https://in.mathworks.com/matlabcentral/fileexchange/70576-baja-all-terrain-vehicle-atv-model.

2) https://en.wikipedia.org/wiki/Baja_SAE.

3) https://www.team-bhp.com/forum/modifications-accessories/52393-baja-atv.html.

4) https://en.wikipedia.org/wiki/All-terrain_vehicle.

5) https://en.wikipedia.org/wiki/Continuously_variable_transmission.

6) https://www.caranddriver.com/research/a31517125/cvt-transmission-vs-automatic-quick-guide/.

7) https://in.mathworks.com/help/physmod/sdl/ref/genericengine.html.

8) https://in.mathworks.com/help/physmod/sdl/ref/doubleshoebrake.html.

9) https://in.mathworks.com/help/physmod/sdl/ref/tiremagicformula.html.

10) https://in.mathworks.com/help/physmod/sdl/ref/vehiclebody.html.